M-1 was a unique submarine, the only one of its class. All submarines up to this point, either Lake or EB designs, had been single hull boats, with all of the ballast tanks and fuel tanks internal to the pressure hull. In a double hull boat, the inner pressure hull was fully wrapped by an outer hull, and the space between the hulls was used for ballast and fuel tanks. The goal was to gain a lot of internal space that could be used for weapons, more powerful engines, and a larger crew. This design by Electric Boat was breaking new ground, as the only other double hull boat in USN service was the Italian Laurenti designed G-4. Because of the lack of experience in building this hull type, the results were not good. The tankage space was very tight and difficult to maintain, and the complicated venting and flooding arrangement made her very unstable when surfacing. She often took large lists until the tanks were fully emptied. M-1 had a short service life, only four years. No further double hulled boats were built for the USN until the T-class fleet submarines of 1920-1922. They were based on an expanded M-class hull and were equally unsuccessful. Double hull and partial double hull submarines were the way of the future, but advances in metallurgy and more experience in building this hull type was needed before it could be successful.
M-1 (Submarine No. 47, later SS-47)
National Archives Photo
M-1 is shown here on builder's trials, probably in Long Island Sound, early 1917. Outwardly she looked a lot like a bigger L-class boat, but she had a radically different hull type. She tended to be a wet boat on the surface, and she became dangerously unstable during the surfacing process.
